CS 103 Unit 14 - Streams

Transcription

1 CS 103 Unit 14 - Streams 1

2 2 I/O Streams '>>' operator reads from a stream (and convert to the desired type) Always skips leading whitespace ('\n', ' ', '\t') and stops at first trailing whitespace '<<' operator used to write data to an output stream 'endl' forces a flush Flush forces the OS to move data from the internal OS stream to the actual output device (like the monitor) input stream (user types all at once): #include<iostream> using namespace std; \t y... int main(int argc, char *argv[]) { cout << "\tit was the" << endl; cout << 4; #include<iostream> using namespace std; int main(int argc, char *argv[]) { int dummy, x; cin >> dummy >> x; input stream: \t y... output stream in OS: \t I t w a s t h e \n 4 output stream after flush: 4 It was the

3 3 Kinds of Streams I/O streams Keyboard (cin) and monitor (cout) File streams Contents of file are the stream of data #include <fstream> and #include <iostream> ifstream and ofstream objects

4 4 When Does It Fail char buf[40]; ifstream inf(argv[1]); fp File text T h e e n d. \n EOF inf >> buf; fp File text T h e e n d. \n EOF For file & eventually stringstreams the stream doesn't fail until you read PAST the EOF inf >> buf; buf fp File text EOF BAD FAIL T h e \ T h e e n d. \n EOF EOF BAD FAIL buf e n d. \ inf >> buf; fp File text T h e e n d. \n EOF EOF BAD FAIL buf e n d. \

5 5 Which Option? #include<iostream> #include<fstream> using namespace std; int main() { vector<int> nums; ifstream ifile("data.txt"); int x; while(!ifile.fail() ){ ifile >> x; nums.push_back(x);... Need to check for failure after you extract but before you store/use data.txt 7 8 EOF nums #include<iostream> #include<fstream> using namespace std; int main() { vector<int> nums; ifstream ifile("data.txt"); int x; while( 1 ){ ifile >> x; if(ifile.fail()) break; nums.push_back(x);... int x; while( ifile >> x ){ nums.push_back(x);... A stream returns itself after extraction A stream can be used as a bool (returns true if it hasn't failed)

6 6 Pattern for File I/O or Streams Step 1: Try to read data (>> or getline) Step 2: Check if you succeeded or failed Step 3: Only use the data read from step 1 if you succeeded If you read and then blindly use the data you will likely get a bogus data value at the end

7 7 Recall How To Get Lines of Text Using the >> operator to get an input string of text (char * or char [] variable passed to cin) implicitly stops at the first whitespace How can we get a whole line of text (including spaces) cin.getline(char *buf, int bufsize); ifile.getline(char *buf, int bufsize); Reads max of bufsize-1 characters (including newline) But getline() uses char* (C-Strings) what if we want to use C++ strings??? input.txt The fox jumped over the log. The bear ate some honey. The CS student solved a hard problem. #include <iostream> #include <fstream> using namespace std; int main () { char myline[100]; int i = 1; ifstream ifile ("input.txt"); if( ifile.fail() ){ // can't open? return 1; ifile.getline(myline, 100); while (! ifile.fail()) { cout << i++ << ": " << myline << endl; ifile.getline(myline, 100); ifile.close(); return 0; 1: The fox jumped over the log. 2: The bear ate some honey. 3: The CS student solved a hard problem.

8 8 C++ String getline() C++ string library (#include <string> defines a global function (not a member of ifstream or cin) that can read a line of text into a C++ string Prototype: istream& getline(istream &is, string &str, char delim); is = any input stream (ifstream, cin), etc.) str = A C++ string that it will fill in with text delim = A char to stop on (by default it is '\n') which is why its called getline Returns the updated istream (the 'is' object you passed in as the 1 st arg) The text from the input stream will be read up through the first occurrence of delim and placed into str. The delimiter will be stripped from the end of str and the input stream will be pointing at the first character after delim. int line_no = 0; ifstream myfile(argv[1]); string myline; while ( getline( myfile, myline ) ) { cout << "Read line: " << myline << endl;

9 STRINGSTREAMS 9

10 10 Introducing Stringstreams I/O streams Keyboard (cin) and monitor (cout) File streams Contents of file are the stream of data #include <fstream> and #include <iostream> ifstream and ofstream objects Stringstreams Contents of a string are the stream of data #include <sstream> and #include <iostream> sstream object

11 11 C++ String Stream If streams are just sequences of characters, aren't strings themselves like a stream? The <sstream> library lets you treat C++ string objects like they were streams Why would you want to treat a string as a stream? Buffer up output for later display Parse out the pieces of a string Data type conversions Very useful in conjunction with string's getline(...)

12 12 C++ String Stream Use << and >> to convert numbers into strings (i.e => "12345") #include<sstream> using namespace std; int main() { stringstream ss; int num = 12345; ss << num; string strnum; ss >> strnum; num getp ss getp ss EOF EOF return 0; strnum "12345" getp ss EOF sstream_test1.cpp

13 13 C++ String Stream Use << and >> to convert strings into numbers (i.e. "12345" => 12345) #include<sstream> using namespace std; int main() { stringstream ss; string strnum = "12345"; ss << strnum; int num; ss >> num; return 0; strnum num "12345" getp ss getp ss getp ss EOF EOF EOF sstream_test2.cpp

14 14 C++ String Stream Can parse a string of many values into separate variables #include<sstream> using namespace std; int main() { stringstream ss; ss << " a" getp ss getp EOF double x, int y; char z; ss >> x >> y >> z; ss a EOF return 0; x y getp ss a EOF sstream_test3.cpp z 'a'

15 15.str() str() member function will return a string with the contents of whatever is in the stream #include<sstream> using namespace std; int main() { stringstream ss; ss << 2.0 << " " << 35; ss << " " << 'a'; string s = ss.str(); getp ss getp ss EOF a EOF return 0; s " a" getp ss a EOF sstream_test4.cpp

16 16 C++ String Stream Beware of re-using the same stringstream object for multiple conversions. It can be weird. Make sure you clear it out between uses and re-init with an empty string Or just make a new stringstream each time stringstream ss; //do something with ss ss.clear(); ss.str(""); // now you can reuse ss // or just declare another stream stringstream ss2;

17 17 Exercise What's in each variable after execution? text num val string text; int num; double val; stringstream ss("hello "); ss >> text >> num >> val;

18 18 Exercises In class exercises Stringstream_in Stringstream_out Date

19 19 Keyboard (use ) Yes Choices Where is my data? File (use ) Do I know how many? String (use ) No

20 20 Choices Is it delimited? Yes Text What type of data? No Integers/ Doubles

21 21 Choosing an I/O Strategy Is my data delimited by particular characters? Yes, stop on newlines: Use getline() Yes, stop on other character: User getline() with optional 3 rd character No, Use >> to skip all whitespaces and convert to a different data type (int, double, etc.) If "yes" above, do I need to break data into smaller pieces (vs. just wanting one large string) Yes, create a stringstream and extract using >> No, just keep the string returned by getline() Is the number of items you need to read known as a constant or a variable read in earlier? Yes, Use a loop and extract (>>) values placing them in array or vector No, Loop while extraction doesn't fail placing them in vector Remember: getline() always gives text/string. To convert to other types it is easiest to use >>

22 22 In-Class Exercises Wordcount

23 23 getline() and stringstreams Imagine a file has a certain format where you know related data is on a single line of text but aren't sure how many data items will be on that line Can we use >>? No it doesn't differentiate between different whitespace (i.e. a ' ' and a '\n' look the same to >> and it will skip over them) We can use getline() to get the whole line, then a stringstream with >> to parse out the pieces int num_lines = 0; int total_words = 0; ifstream myfile(argv[1]); string myline; while( getline(myfile, myline) ){ stringstream ss(myline); string word; while( ss >> word ) { total_words++; num_lines++; double avg = (double) total_words / num_lines; cout << "Avg. words per line: "; cout << avg << endl; The fox jumped over the log. The bear ate some honey. The CS student solved a hard problem.

24 24 Using Delimeters Imagine a file has a certain format where you know related data is on a single line of text but aren't sure how many data items will be on that line Can we use >>? No it doesn't differentiate between different whitespace (i.e. a ' ' and a '\n' look the same to >> and it will skip over them) We can use getline() to get the whole line, then a stringstream with >> to parse out the pieces Text file: garbage stuff (words I care about) junk vector<string> mywords; ifstream myfile(argv[1]); string myline; getline(myfile, myline, '('); // gets "garbage stuff " // and throws away '(' getline(myfile, myline, ')' ); // gets "words I care about" // and throws away ')'` stringstream ss(myline); string word; while( ss >> word ) { mywords.push_back(word); mywords "words" "I" "care" "about"